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#1
25-03-2010, 08:55 PM
 browndog (Ian) Registered User Join Date: Jan 2007 Location: Perth Posts: 129
How do you plan a mosaic image?

I am thinking that I would like to try a mosaic image sometime soon. This would allow me to image widefield targets that are too large for my current equipment.

I was wondering how others go about planning their mosaic images. In particular, how do you calculate the centre of each tile in the mosaic based on the desired size of the final combined image?

Is there free or simple software that can automate this, or do you calculate manually?
#2
25-03-2010, 09:48 PM
 Octane (Humayun) IIS Member #671 Join Date: Dec 2005 Location: Canberra Posts: 11,158
Ian,

Dr. Robert Gendler has written an article on this very topic.

http://www.robgendlerastropics.com/Compositions.html

H
#3
25-03-2010, 10:05 PM
 mithrandir (Andrew) Registered User Join Date: Jan 2009 Location: Glenhaven Posts: 4,161
Quote:
 Originally Posted by browndog I am thinking that I would like to try a mosaic image sometime soon. This would allow me to image widefield targets that are too large for my current equipment. I was wondering how others go about planning their mosaic images. In particular, how do you calculate the centre of each tile in the mosaic based on the desired size of the final combined image?
Traditionally mosaics are done with about 30% overlap. You can probably get away with less on astro images, as long as your field is pretty flat.

Quote:
 Originally Posted by browndog Is there free or simple software that can automate this, or do you calculate manually?
If you know your arcseconds/pixel you can work backwards from that. e.g QHY8+0.33FR on a C8 is 206.265/(2032*0.33)*7.8 or about 2.4 as/p, so the smaller dimension FOV is roughly 2000*2.4 = 4800 arcsec, with 30% overlap that gives 4800*0.7 or 3300 arcsecs you can move between frames. Someone with more experience might want to check the math.

To automate the stitching, Hugin is free, or if you have a few hundred spare Euros, Autopano Professional. You might find you need to process each image separately first.

Quote:
 Originally Posted by Octane Dr. Robert Gendler has written an article on this very topic.
Interesting, but maybe overkill for someone just wanting to try mosaics. Wouldn't you need a permanent setup to make full use of Rob's method?
#4
25-03-2010, 10:22 PM
 Octane (Humayun) IIS Member #671 Join Date: Dec 2005 Location: Canberra Posts: 11,158
Andrew,

Yeah, no doubt -- but, there's stuff in there that anyone can take away and use. For example, the whole screenshot and composition techniques.

Also, people like Jase use 10% overlaps, from memory. You don't want such a large overlap that the blending become really visible due to having more data in the overlapping regions, and, what not. I've never done an astro one so I might be wrong, but, am going by terrestrial experience.

H
#5
26-03-2010, 07:20 AM
 mithrandir (Andrew) Registered User Join Date: Jan 2009 Location: Glenhaven Posts: 4,161
H,
Quote:
 Originally Posted by Octane Yeah, no doubt -- but, there's stuff in there that anyone can take away and use. For example, the whole screenshot and composition techniques.
I'm not disputing that. When learning to swim you don't jump in the open ocean.

Quote:
 Originally Posted by Octane Also, people like Jase use 10% overlaps, from memory. You don't want such a large overlap that the blending become really visible due to having more data in the overlapping regions, and, what not. I've never done an astro one so I might be wrong, but, am going by terrestrial experience.
I'd have to ask what it is you are using to do your stitching. I've used Autostitch - from where it all began - and have Autopano.

Autopano does field curvature corrections for the focal length of the lens. It then picks out common points to overlay the frames. Depending on the subject you might need more overlap to get good matches. When it renders you can choose the blend options and it deals with modest changes in light levels.

My Warrumbungles shot here http://www.iceinspace.com.au/forum/s...57&postcount=1 has about 50% overlap between frames. There is about a minute between shooting the leftmost and rightmost edge.

The only real complaint with it is it doesn't handle FITS, but you'd have to debayer or stack your mono frames first anyway.

Andrew

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